Intercontinental Ballistic Missiles

  
During the last months we heard (and read) a lot of news about ballistic missiles and potential threads of North Korea testing Intercontinental Ballistic Missiles (ICBM). But what is an intercontinental ballistic missile and why does it pose such a threat?
          
First, it is important to understand what a ballistic missile is. Basically, there are 2 types of missiles: ballistic missile and cruise missiles (will be explained in another post).
A ballistic missile is a missile that follows a ballistic trajectory; that is, the path that any given thrown object (or projectile) without propulsion would take under the action of gravity. A theoretical perfect projectile trajectory only considers gravity but real projectiles also consider additional forces such as friction due to aerodynamic drag (Figure 1).
Fig 1. Ballistic trajectory

The flight of a ballistic missile has 3 phases: A first phase is a powered flight (launching); in long range missile the launching requires enough force so that the missile escapes the atmosphere. The second phase which is the longest phase is the free flight (a ballistic projectile). Usually, this second phase is outside of the atmosphere; hence, the missile has less friction. The third phase is the re-entry phase.
Range
Ballistic missiles can be divided in four groups based on their range (Figure 2):
Short range ballistic missile. This missiles have a range between 300 km to 1000 km and may reach their target in less than 10 min.
Medium range. This missiles have a range between 1000 km and 3500 km and may reach their target in between 10 min to 15 min.
Intermediate range. This missiles have a range between 3500 km and 5000 km and may reach their target in between 15 min to 20 min.
Intercontinental range. This missiles have a range greater than 5000 km (up to 13000 km) and may reach their target in between 20 min to 30 min.
Fig 2. Range of ballistic missiles if launched from North Korea

The thread
ICBM are big threat to world's peace because of 2 factors: their range and short time to reach their target.
Range. Ballistic missiles flying above the atmosphere have a range much longer than any other missile. ICBM may easily reach target distances about 10000 km.  That is, a missile launched from North Korea may reach any European capital, any Asian city, Australia or even some major American cities such as Seattle, Los Angeles, Las Vegas or Houston.
Time to target. ICBM may reach velocities about 5 km/s and may require only 30 min to travel up to 10000 km. That is, any of the previously mentioned cities could be destroyed in just 30 min after the launch is detected. With such a short time, there would be no option to evacuate or to appropriate shelter. Thus, ICBM are lethal.

Fig 3. Ballistic missiles

This short introduction to ICBM clearly illustrates their potential as destructive lethal mass destruction weapons. Although America is developing-testing a missile defensive system, ICBM are still a threat